Coating composition containing polysiloxane resin and polysiloxane rubber, film coated therewith and process of making



tam al e- COATING COMPOSITION CONTAINING POLY- SILOXANE RESIN AND POLYSILOXANE RUB- BER, FILM COATED THEREWITH AND PROCESS OF MAKING Philip Strubing Blatz, Bufialo, N.Y., assignor to E. I. ,du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Filed July 18, 1955, Ser. No. 522,819,

' 12 Claims. (cl. 260-37) 2-10, inclusive, is fully disclosed in United States Patent No. 2,465,319 to Whinfield & Dickson. From a commercial standpoint, one of the most attractive polymers of this class is polyethylene terephthalate.

Polyethylene terephthalate film, particularly oriented film (film which has been stretched and/or rolled in two directions and heat-set at elevated temperatures within the range from 150-250 C.) has been found to possess a unique combination of electrical, physical, and chemical properties which make it outstanding for use as a dielectric. This is particularly true of polyethylene terephthalate film which has been stretched and/or rolled to substantially the same degree in both directions, e.g., 3X, where X equals the original dimension of the film, to obtain a substantially balanced" film, which means that the physical properties of the film as measured in both directions are substantially thesame. The oriented, heatset polyethylene terephthalate film is outstanding as a dielectric because it retains a considerable percentage of its electrical properties, strength, and durability at elevated operating temperatures. Consequently, the film is particularly, useful as a dielectric in capacitors, motors, generators, transformers, etc., at not only moderate operating temperatures, but also at ambient temperatures approaching 150175 C.

In extending the usefulness of polyethylene terephthalate and like thermoplastic polymeric film dielectric to .a wider variety of electrical end uses, it became apparent that the di lectrici life of the film required improvement, particularly for use in equipment subjected to high voltage stresses. -As employed herein, the term -dielectric life? of the polymeric thermoplastic film applies to the actual time that a particular film may be subjected to a particular voltage stress under conditions of corona discharge before actual physical breakdown, i.e., rupture, of the film dielectric,

It is the action upon the polymeric film dielectric of corona discharge (in air or other gaseous medium) which causes actual physical breakdown of the film under conditions of a particular voltage stress (a potential difference large enough to produce a visible discharge but not large enough to produce instantaneous breakdown). Obviously, the lower'the voltage stress, the longer the dielectric life of the film. Corona discharge is defined as the discharge f electricity which appears upon the surface of a c nduc o w e t e p t n l adien exceeds a cer- For example, when a continuous potential applied to a pair of narrow wires is slowly increased, the voltage will be'reached at which a hissing noise is heard, and a pale violet light is visible "(in the dark) around United States Patent O Ice the wires. This voltage is defined as the critical visual corona point? Corona is due to ionization of the air or surrounding gases (it does not occur in a vacuum). That is, the air in the ionized region is conducting, and this results'in increasing the effective diameter of the conductor. For example, even though a film dielectric is wound tightly around a wire, air is not entirely excluded from the space between the conductor and the film dielectric; and it is the air or other gaseous medium which is ionized. Thecorona envelops a conductor as a concentric cylinder, and the outside diameter becomes .such that the gradient at that point decreases to the rupturing point of the air. Beyond this point, the corona cannot increase for the constant applied voltage because the gradient decreases with increasing radial distance from the wire. The glow or breakdown of the surrounding air starts first at the point of maximum gradientor atthe conductor, surface.

An object of the present invention is to provide a polymeric thermoplastic dielectric film having increased dielectric life. A further object is to provide a polyethylene terephthalate film dielectric having increased dielectric life. A further object is to provide a process of improving the dielectric life of polymeric thermoplastic dielectric compositions, particularly polyethylene terephthalate film. A still further object is to provide a novel coating composition useful for improving the dielectric life of polymeric thermoplastic dielectric film. Other objects will be apparent from the following description of the invention.

These objects are realized in accordance with themesent invention which, briefly stated, comprises providing a dielectric base film of thermoplastic polymeric ma terial, e.g., polyethylene terephthalate, with a relatively thin adherent coating of a cured homogeneous blend comprised essentially of a polymeric silicone resin and a polymeric silicone rubber, said blend containing uniformly distributed finely divided particles from the group consisting of amorphous silicaand estersils.

The blend of silicone resin, silicone rubber, and finely divided particles may be prepared by starting with a heterogeneous mixture of the three individual components, and blending, e.g., ball milling, them together. On the other hand, the blend'may be prepared by starting with a heterogeneous mixture of a silicone resin and a preformed blend of finely divided amorphous silica particles or estersil particles in a silicone rubber, e.g., a commercially available silica-filled silicone rubber, and: blending the components together in the presenceof a finely divided phthalocyanine pigment of the type to berde scribed hereinafter. I

The present invention will be further described sp'e- 'cifically with respect to improving the dielectric life of polyethylene terephthalate film, but it should be under-1 stood that the invention is -applicable as well to enhanc ing the dielectric life of other types of unplasticized poly meric thermoplastic films useful as dielectrics, such as films of polyethylene, polytetrafiuoroethylene, polystyv rene, polyamides, etc.

, I 2,977,336 Patent-ed Mar. 28,, 196,1

oxyaryl or aroxyalkyl silicone resin.

as methyl silicone resins, are described and claimed in United States Patent Number 2,258,218. These heathardenable methyl polysiloxane resins may be prepared,

for example, by hydrolysis of suitable mixtures of methyl silicon halides, or mixtures of methyl silicon halides and a silicone tetrahalide, in such proportions that the resulting methyl polysiloxane resins contain an average of from 1 to substantially less than 2, preferably 1.3 to 1.7 methyl groups per silicon atom. The liquid hydrolysis and condensation products are resinous materials which may be further condensed or polymerized to a solid state.

Depending upon the nature of the starting materials employed in preparing, for example, a methyl silicone resin, the polymer chains are generally made up of the following types of unit structures. The dimethylsiloxy p.

tit-ocomprises a considerable portion of the polymeric chains, particularly when the methyl-to-silicon ratio approaches 2. Generally, the above structural unit is assumed to include half of each of the two associated oxygen atoms and, hence, is more properly written,

This structural unit is found as a terminal unit of a polymer chain in that the methyl groups are not capable of further condensing.

As mentioned above, other groups or radicals may replace the methyl groups in the above illustrations of typical structural units, and the resulting siloxane resins take on diiferent characteristics. For example, United States Patent Number 2,258,220 describes and claims ethyl polysiloxane resins (ethyl silicone resins). The structure of these resins comprises structural units similar to those illustrated above except that the, ethyl group replaces the methyl group.

United States Patent Number 2,258,221 describes ar- Here again, the general structure of these resins contains structural groups similar to those given above except, for example, the methyl radicals are replaced with aroxyaryl or aroxyalkyl radicals.

United States Patent Number 2,258,222 describes still another type of silicone resin wherein one methyl radical in the structural units given above is replaced with an aryl group, such as a phenyl group or a halo-aryl radical. On the other hand, another useful group of silicone resins are the aryl silicone resins or aryl polysiloxane resins wherein all methyl groups in the above-illustrated structural units are replaced with aryl groups, such as phenyl groups.

A considerable number of techniques and processes have been developed for preparing the silicone resins and three halogenosilane.

of the more common procedures, described with respect to preparing methyl silicone resins, are as follows:

(1) Dimethyl silicone may first be prepared by bydrolyzing dimethyldichlorosilane or its esters, and then oxidized with air and a catalyst in accordance with the process described in United States Patent Number 2,438,- 478 to attain the desired methyl-to-silicon ratio.

(2) Dimethyldichlorosilane may be mixed with meth' yltrichlorosilane or silicon tetrachloride and then bydrolyzed and the products co-condensed.

(3) Silicon tetrachloride may be partially methylated (as with methyl magnesium chloride) to the desired methyl-to-silicon ratio and the reaction mixture hydrolyzed directly as illustrated in United States Patent Number 2,258,218.

The methyl-to-silicon ratio of the final resin product is normally dependent upon the nature of the initial reactants or raw materials and the conditions of the condensation reaction. The closer the methyl-to-silicon ratio is to 2, the lower the degree of cross-linking in the resin structure; and as the methyl-to-silicon ratio approaches 1, the greater the degree of cross-linking in the resin structure. In general, it appears desirable to em ploy silicone resins in the present coating compositions having an R/Si ratio from about 1.3 to 1.8, where R stands for any of the types of substituent groups, or combinations thereof, mentioned above, that is, methyl, ethyl, phenyl, etc. For example, when a methyl phenyl silicone resin is used, it is normally preferred to have a higher proportion of methyl groups than phenyl groups in the resin in order to avoid brittleness, for example, a resin having an average of 1.00 methyl group and 0.80 phenyl group per silicon atom.

In the present invention it is preferred to employ a substantial quantity of the flexible silicone resins. For

example, the methyl phenyl silicone resins have been found to be highly desirable from the standpoint of flexibility, these resins being more flexible than the dimethyl silicone resins and the diphenyl silicone resins. In general, it is preferable to employ a mixture of silicone resins, e.g., 50% of a flexible methyl phenyl silicone resin and 50% of faster-curing, but more brittle, dimethyl silicone resin. It is preferred that at least 50% of the mixture be a flexible silicone resin.

Commercially available silicone rubbers or silicone gums which may be blended with silicone resins and finely divided particles to form the present coating compositions are generally methyl siloxane polymers having practically no chain branching or cross-linking and an average molecular weight of 400,000 to 500,000, and in some cases as high as 1,000,000. The silicone gums in general are a water-white material having an extremely high viscosity (10,000,000 to 12,000,000 centistokes).

In the preparation of the methyl silicone gums or rubbers (described inUnited States Patent No. 2,448,756 to M. C. Agens), the dimethyl silicones found suitable for the preparation of materials possessing elastomeric properties are those obtained by hydrolysis of a pure or substantially pure dimethyl dihalogenosilane, such as dimethyldichlorosilane or an equivalent methyl-substituted silane, such as dimethyl-diethoxysilane, containing two methyl groups and two hydrolyzable atoms or groups connected to the silicon atoms. In the case of the methyl silicone gums or rubbers, as mentioned hereinbefore, the polymer structure contains practically no branching or cross-linking, and therefore the resulting products contain a ratio of methyl-to-silicon atom of two or substantially two. For example, the dimethyl silicone gums having a methyl-to-silicon ratio from 1.98 to 2.00 may be prepared from the hydrolysis product of dimethyl dihalogenosilane alone, or a dimethyldihalogenosilane containing not more than 2 mol percent of a methyltri- The resulting products are normally divided into two groups, (1) the soluble gums having a methyl-to-silicon ratio of 2, and (2) the insoluble gums having a r'nethyl-to-silic'on ratio' of at least: 1.98 but5 less than 2. In general, the insoluble gums require morev extensive masticating or milling with the silicone resin in the presence of a solvent in order to obtain homoge nous solutions.

Another useful type of silicone rubber which is curr'ently commercially available is the' methyl phenyl siloxane polymer (described in United States Patent Number 2,484,595 to M. M. Sprung) wherein part of the methyl groups along the siloxane chain are replaced with phenyl groups. In these rubbers the methyl plus phenyl-to-silicon ratio is also between 1.98 and 2. Other rubbery polymeric organo-siloxane compositions which may be employed in the present coating compositions include those wherein the methyl groups in the dimethyl silicone gums or rubbers are replaced by other alkyl and/or alkylene and/or aryl groups, e.g., ethyl silicone gums.

Types of-finely divided particles which a're'to be blended with the blend of silicone resin and silicone rubber are selected from the following compounds:

(1) Amorphous silica, and

(2) Estersils as defined in USP 2,657,149.

The essential requirement is that whenever any of the above compounds are blended with the silicone resin/silicone rubber blends the compound must be in finely divided form such that the-ultimate particles, either in the form of super-colloidal aggregates or individual particles, have an average greatest dimension ranging between 5 millimicrons and 5 microns, and preferably below 50 millimicrons.

Some types of the above compounds may not be commercially available in finely divided form wherein the average particle size is 5 microns or less. In such cases it'is necessary to process the particles by ball milling or other techniques to reduce the size of the average particle.

A' considerable number-of commercially available types of amorphous silicas may be satisfactorily employed in the present invention, and these silicas may be arbitrarily divided into four classifications: aerogel, precipitated, vapor phase and diatomaceous types. Santocel" CS, manufactured by the Monsanto Chemical Company, is an example of the aerogel type. This is slightly acidic and contains small amounts of alcohol, water, and sodium sulfate as the chief impurities. The particle diameter is approximately 30 millimicrons, with a surface area of approximately 110-150 m. gm. (square meters/ gram). The precipitated silicas may be represented by Hi-Sil X303 or Hi-Sil XW, manufactured by the Columbia-Southern Chemical Corporation. This material has an average particle diameter of 20-25 millimicrons and a surface area within the range of 140-160 mF/gm. A typical representative of the vapor-phase type of silica is Aerosil, manufactured by Godfrey L. Cabot, Incorporated, this material having an average particle diameter approximately 15-20 millimicrons and a surface area of 175-200 mfi/gm. The diatomaceous types of silicas may be further subdivided into two groups which are important in compounding the present coating compositions. These are the heat-calcined and fiuxedcalcined materials. Both are relatively free from organic contaminents and have no adverse eifect upon vulcanizing systems usually employed. Particle sizes are much larger than those of the silicas mentioned above, ranging from 1 to 6 microns. The surface areas are larger than would be calculated from particle diameter owing to high internal area in this form of silica. Inthe, heat-calcined group are such products as'Celite 270, manufactured by Johns-Manville Corporation, and Dicalite? PS, manufactured by Great Lakes Carbon Corporation. In the flux-calcined group are Dicalite? White, and Celite Super Floss; v

Estersils, described in United States Patent Number 2,657,149, to Ralph K. Iler, embrace a class of compounds which are preferred components in thepresent coating Jcompositionsi" In brief,lthe1estersils arefi-e'sterified super-colloidalsubstrates; that is, a'substrate' in the form of particles coated with OR" groups, the substrate having a surface of silica and h'avinga specific surface area of at least 1 m. /gm., the'coatin'g of OR groups being chemically bound to said silica, and R" being a hydrocari bon radical of atleast. 2' carbon atoms. wherein thecarbon atom attachedto oxygen is also attached to hydrogen.

In preparing the coating composition of the present invention, i.e., one containing finely divided particles'of amorphous silica or finely divided particles of estersil, it is convenient to employ a commercially available silica-filled or estersil-filled silicone rubber for blending with the silicone resin. It-must be emphasized atthis point thatto obtain a homogeneous blend of a silicafilled'silicone rubber, e.g., having silica particles or estersil particlesuniforml-y dispersed throughout, and a silicone resin of the type described hereinbefore, it has been found that the blending or mixing of'the two must be carried out in-the pre'sence'of a phthalocyanine pigment. That is, to'apply a firmly adherent, uniform, non-tacky silicone resin/silicone rubber/silica or estersil coating uponthe surface of a dielectric film, the coating composition itself must be thoroughly and homogeneously blended; and it has been found that'coating compositions prepared by merely mixing, e.g.', ball milling, the silicone resin and the filled silicone rubber together'in a common solvent arenot capable of forming a homogeneous cured coating upon a polymeric film surface. Therefore, it has been found that by blending thes'ilicone resin and the silica-filled silicone rubberin a solvent in the'presence of a chlorine-free phthalocyanine pigment-of the type defined in United States Patent Number 2,556,726 to F. W. Lane, the resulting blend'rnay be readily applied as a coating to the surface of a polyethylene 'terephthalate film to produce a hard, non-tacky; flexible, homogeneous coating upon the filmsurface after the coating composi tion has been cured. Thechlorine-free phthalocyanine pigment apparently functions as a dispersing or compatibilizing agent for providing: homogeneity and thereby makes possible the formation of'a silicon resin/silicone rubber/silica or estersil blend (when starting with blend ing a silicone resin and a filled silicone rubber) capable of providing a hard, flexible non-tacky coating upon the surface of thermoplastic, polymeric films useful as elec phthalocyanines of metalsother than copper, all of the phthalocyanines beingsubstantially free of chlorine.

If the present preferred coating compositions are to e be prepared by directly blending a silicone resin, a silicone 1 gum or rubber, and finely divided particles of silica or estersil, that is, the silicone gum or rubber not having silica or estersil particles predispersed therein, it should be emphasized that it is not necessary to employ the phthalocyanine pigment as a compatibilizing'agent. In

other words, the three individual essential components,

the siliconeresin, silicone rubber, and the silica particles or the estersil particles, may beadded individually to a solvent, such as xylene, and the'mix'ture then blended in accordance with known techniques" to form a homoge neousblend or mixture of the compon'ents. In attempting -toapply coating compositions to polymeric' thermoplastic film's useful as dielectrics for the I purpose of increasing the dielectric lifefof a particular film, anumb'er of influencing factors rnus't be taken -intO consideration. That is, (l) the coating should not impair any of the desirable physical, chemical, or electrical properties of the base film (for example, the coatingshould be substantially as flexible and non-tacky as the base film), (2) the coating must be capable of being applied homogeneously overthe entire film surfce in order that pinholes in the coating are not formed, and (3) the coating should be capable of being cured at moderate temperatures in order that the base film would not have to be subjected to excessively high curing temperatures. The present silicone resin/rubber blend described herein fully meets these requirements. Furthermore, this blend, even when applied as a very thin coating (less than 1 mil thicknesses on both sides of dielectric base film) is highly effective in materially extending the dielectric life of useful dielectric films, particularly at elevated temperatures. Other types of coatings, such as silica-filled silicone rubber (with no silicone resin) must be applied as heavy coatings to dielectric films to produce a product having a dielectric life equivalent to that attainable with the coated films of this invention. On the other hand, heavy coatings are to be avoided because they impair many of the desirable physical properties of the dielectric base film, particularly flexibility. In general, the silicone rubber lends flexibility to the coating composition; and the silicone resin composition facilitates curing the coating composition at relatively moderate temperatures which will not affect the properties of the base film, e.g., polyethylene terephthalate film.

In compounding the blend of silicone resin/silicone rubber/finely divided particles of the present invention, the following proportions of components are preferred (percent by weight of total solids): silicone resin, 40- 75%; silicone rubber, 15-40%; finely divided particles of amorphous silica or estersil, 520%; and phthalocyanine pigment, 0-15 The criticalness of the concentration of the finely divided compounds blended with the silicone resin/silicone rubber composition is illustrated in the examples to be presented hereinafter. In general, the smaller the average particle size of the finely divided amorphous silica particles or estersil particles, the lower the concentration (by weight) necessary to produce a satisfactory coating for dielectric base film. When the concentration of finely divided particles exceeds 20%, by weight, of the total weight of the coating composition (resin/rubber/ particles), the corona resistance of the resulting coated dielectric films appears to decrease and in general, the optimum coating compositions of this invention contain no more than about 20% of the finely divided compound.

As a general observation, it has been found that the degree of corona resistance of any given dielectric base film coated with the compositions of the present invention depends upon the extent of cure of the resin/rubber/ particles coating, the amount (by weight) of the finely divided particles, the average particle size of the finely divided particles, and the thickness of the coating. The more complete the cure, the greater the corona resistance (dielectric life) of the coated film; and the greater the coating thickness, the greater the corona resistance.

The final step in providing the thermoplastic dielectric film, e.g., polyethylene terephthalate film, with a nontacky uniform coating of the subject silicone resin/ silicone rubber/finely divided particles is the curing of the resin/rubber blend at moderate or elevated temperatures. Normally, curing agents have already been incorporated into the resin and rubber compositions when these compositions are obtained from the manufacturer. The curing agents already present in the compositions may or may not be satisfactory alone; hence, it may be necessary to add an additional curing agent to the coating composition in order to accelerate the rate of cure at any selected temperature. Most of the curing agents are designed to promote relatively rapid curing at moderately elevated temperatures; and it is important in 'plastic film dielectric.

the presentsituation to be able to effect curing at tem peratures which, will not adversely affect the physical, electrical, and chemical properties of the base thermo- Numerous types of curing or vulcanizing agents may be employed and these are specifically listed and described in the prior art. Additional quantities of the curing agents may be added to the resin/rubber coating compositions in the form of solutions of the curing agents in the solvent, such as the same solvent in which the resin and rubber are to be blended, e.g., toluene, xylene, etc. Various curing agents which may be employed for the silicone resins include lead salts, such as lead naphthenate, lead octoate, zinc octoate and benzoyl peroxide. Benzoyl peroxide may also be' employed as a curing agent for the silicone rubber, as described in United States Patent Number 2,448,565 to Wright and Oliver. Organic metallo compounds are also useful as curing agents for silicone rubbers and for the silicone resin/rubber blends of the present invention. These organic metallo compounds include triphenyl stilbine, dimethyl mercury, tributyl tin acetate, tetraethyl lead, tetraphenyl bismuth, lead tetraacetate, and mercury acetate, described in United States Patent Number 2,480,620 to Warrick. Other organic metallo compounds which appear to be useful in promoting curing of the present silicone resin/rubber blends include organic compounds of titanium such as the organic acyl polytitanates, such as isopropyl oleoyl polytitanate (polymeric isopropyloxy titanium oleate) which may be prepared in accordance with the process described in United States Patent Number 2,621,195 to Haslam. These organic titanium compounds serve to promote rapid curing of the silicone resin/rubber blend, particularly when employed in the presence of a small amount of an additional compound which promotes hydrolysis of the titanium compound such as red iron oxide powder.

The present coating compositions are conveniently applied to thermoplastic polymeric base films from solutions thereof in solvents in which the blending of silicone resin/silicone rubber/finely divided particles has been carried out. Thus, in compounding and preparing the present coating compositions for application to the base dielectric film, suitable quantities of the silicone resin (normally in the form of a viscous solution of the silicone resin in a solvent), the silicone rubber (normally in the form of a gel-like mixturewhich includes a solvent), and finely divided particles of amorphous silica or estersil are added to a suitable quantity of a volatile organic solvent for the resin and the rubber, such as toluene, xylene, carbon tetrachloride or chloroform. As mentioned hereinbefore, other materials such as additional curing agents may then be added. The mixture of materials is agitated with the solvent medium in a suitable mixing vessel; and the insoluble components, e.g., silica particles or estersil particles are uniformly suspended in the solution.

The coating composition may be applied to one or both surfaces, preferably both surfaces, of the base film by any desired expedient; and the coated film may thereafter be dried at room temperature or a moderately elevated temperature to remove solvent. Curing of the blend of silicone resin/rubber may well be effected at room temperature, depending upon the curing agent, and in other cases may be carried out at elevated temperatures, e.g., l00l75 C., for durations from 15 minutes to 60 minutes.

It may be necessary, depending upon the end use for the coated dielectric films of the present invention, to apply a subcoating to the base dielectric film, e.g., oriented heat-set polyethylene terephthalate film, to improve adhesion between the base film and the silicone resin/ rubber coating. When applying the subject coating compositions -to polyethylene terephthalate film, it is preferred to apply a subcoating which is chemically similar to the base film.

Any suitable subcoatings may be employed provided now-pee that they improve the adhesion between the base film and the present coatings and do not materially adversely atfect the original combination of. physical, electrical, and chemical properties of the base film. Preferred subcoatings include copolyesters derived by reacting glycol, terephthalic acid, or low alkyl ester thereof with a second acid or alkyl ester thereof from the group consisting of sebacic acid, isophthalic acid and hexahydroterephthalic acid. Normally, in preparingsuitable subcoatings it is preferred that the subcoating'composition contain at least 50% of the terephthalic acid component, based upon the total weight of acid components. The subcoating compositions maybe applied to the base dielectric film from solvent solutions, or homogeneous thin films of the copolyester compositions may be first applied to the dielectric base film by lamination under heat and moderate pressure.

The following examples of embodiments of the present the invention. Parts are by weight.

EXAMPLE 1 The following formulation: Methyl phenyl silicone resin (General Electric SR-17) a 4 parts. Dimethyl silicone resin (General Electric SR-98) 4 parts. Dimethyl silicone rubber gum (General Electric SE-76) 2 parts.

Estersil (described in USP 2,657,149) 1 part. Lead octoate (catalyst to promote curing) 3% (based on total weight of resin component). (Resin 72.7%, rubber 18.2%, estersil 9.1%.)

was dissolved and dispersed in toluene to form a composition containing 18% solids. This composition was placed in a container partially filled with glass beads, and the container was agitated on a rolling mill until a uniform dispersion of the non-soluble component (the estersil) was obtained. Oriented (stretched 200% in both directions) heat-set (at a temperature of about 200 C.) polyethylene terephthalate film was employed as the base film dielectric. The base film /z milin thickness) was first coated with a thin coating (about 1 micron in thickness) applied from a solution of toluene containing 1 part of a methyl phenyl polysiloxane resin (General Electric SR-32) and 2 parts of tetraisopropyl titanate (to form a solution containing 2% solids in toluene). The

base film was passed through the solution, most of the coating was wiped oif, and the coated film was heated in the presence of moisture.

The subcoated film was then passed through a solution of the silicone resin/silicone rubber blend and then be tween doctor rolls to remove excess coating composition. The coating was dried at room temperature for 1 hour. Thereafter, the coating was cured at 150 C. for 15-30 minutes. The resulting coating was smooth, non-tacky, and firmly adherent to the base polyethylene terephthalwas blended together in a container partially filled with glass beads, .and the container was agitated on a rolling for -30 minutes.

mum about: 2-4 hours. Oriented (stretched ax in: both directions) heat-set (at :a temperature of about 20006:) a i polyethylene terephthalate. film .was employed aslthe' base film dielectric. The base film (/2 mil in thickness) was first coated with a thin subcoating (less than 1 gm.'/;m.

of a copolyester of ethyleneterephthalate/ethylene.seb-

acate (60/40) from a solvent (chloroform) solution,

and the solvent was. permitted to evaporate at room tern;

perature. The subcoated film was then passed through a solution of the silicone resin/rubber blend and then. between doctor rolls to remove excess coating composition. The coating was dried at room temperature for 1 hour. Thereafter, the coating was cured at 150 C. The resulting coating was smooth, non-tacky and firmly adherent to the base polyethylene terephthalatefilzn.

The above example was repeated except that the hf Phthalocyanine"pigment was omitted from the coating invention further illustrate the principles and practice of formulation. The final cured coating, which was not homogeneous, onthe base film was somewhat sticky and was easily rubbed off by rubbing ones finger across the coated film surface.

' omitted from the coating composition.

EXAMPLE 4 Example 1 was repeated except that the silicone resin was omitted from the composition and particlesof an amorphous silica were substituted for the estersil. The I composition was composed of 2 parts of dimethyl -sili'- cone rubber gum to 1 part of amorphous silica particles.

Benzoyl peroxide (3%. based on'weight .of'rubber) was; employed as the curing catalyst. The silica particles used were Aerosilmanufactured by Godfrey L. Cabot, Inc.

EXAMPLE 5 Example 1 was repeated except'that the resin, rubber and estersil were blended in the following proportions:

- "Parts Methyl phenyl silicone-resin (GE SR-1-7) 1 "2 Dimethyl silicone resin(GE SR-98) 2 Dimethyl silicone rubber (GE SE-76) 1 Estersil ..1

(66.7% resin, 16.65% rubber, and 16.67% ;estersil) EXAMPLE 6 Example [5 was repeated except that benzoyl peroxide was used in place of lead octoate as the curing catalyst.

Dielectric life test The fi m sample to be tested was placed on -a-brass plate 7 to which the'high voltage was to be applied. Abrass rod (6" long and A1" in diameter) was placed on top of the fim sample and normal to the brass plate (thefilm sample Was sandwiched between the brass plateandan end of the brass rod). Therodserved as the ground electrode. The endo-f the rod touching the film sample was v,

rounded off at a radius of curvature of 5 The pressure exerted on the film was due only to the gravitational forces on the rod. The entire test apparatus was, set up in air. Sufficient voltage was applied to the plate to give Q U in the flow of current was used to tripv a relay giving a record of the :failure. Ten samples were treated simul taneously. The time to gthfi failureof the fifth tSjfi glP lg ,vvas

Reaction product of glycol and 60 parts dimethyl terephthalate-and .40 partsidlmethyl sebacate.

Furthermore, for the same finely divided particles (same average particle size) in any given resin/rubber blend, the dielectric life (of a film coated with the composition) begins to decrease when the concentration of particles in the composition is increased appreciably above 20% (by weight of the total composition).

TABLE IL-DIELECTRIC LIFE (CORONA RESISTANCE) OF POLYE'IHYLENE TEREPI-I MIL IN THICKNESS COATED WITH SILICONE RESIN/SILICONE RUBBER/PARTICLES COMPOSITION Average Weight Wt. Pcr- Total Film Size f Ratio of cent f Total Film Dielectric Example Name and Type of Particles Particles Rcsin/ Particles Thickness Li ie at 1.000

(microns) Rubber] in Coating mils) Volts/Mil Particles (hours) Control... none. 0. 5 5-7 7 Estersil (described in U.S.P. 0.05 4/1/05 9.1 1.3 304 do 0.05 4/1/1 16.7 1.0 48 do 05 4/l/l 16.7 1.1 231 do 0. 5 4/1/1. 5 23.1 1. 107 ---..do 0. 4/l/2 28. fi 1. 0 29 0.5 4/1/2 5 33 3 1.0 21 0 5 411/3 37.5 1.0 14 Vapor phase type (Cnb-O-Sil")- 0.015 4/1/0. 5 9.1 1.1 220 do 0 015 4.'l,'1 16.7 1.6 159 Precipitated typc ("Hl-Sil" l01). 0 025 4/1/1 l6. 7 1. 5 430 Aeragel type ("SantoccP' CS)... 0.030 4/10 16. 7 1.0 264 Dg ltgmaceous type (Celite" 1.0 4/1/1 16. 7 1.0 317 Diatomaceous type ("Celite 2.0 4/1/1 16. 7 1.1 265 Superflnss). Aerogel type (Syloid" 244)--. 3.0 4/111 16. 7 1.2 430 ness of the coated film (the base dielectrlc film was 0.5 EXAMPLE 21 mil in thickness). Table I summarizes Examples l-6, inclusive.

TABLE I[.-DIE EC'IRIC IFE (CORONA RESISTANCE) OF PO YETHYLENE TEREPH'IHA' ATE FIT/M MI IN THICKNESS) COATED WITH SI' ICONE RESIN/SILICONE RUBBER/PARTICLES COMPOSITION In all of the following Examples 7-20, inclusive, the components of the coating composition were blended together in the following proportions:

Methyl phenyl silicone resin (GE SR-l7) 2 parts. Dimethyl silicone resin (GE SR-98) 2 parts. Dimethyl silicone rubber (GE SE-76) 1 part. larticles /2-3 parts. Lead octoate (catalyst) 3% (based on weight of resin).

In every case the coating was applied to a /2 mil oriented, heat-set polyethylene terephthalate film. Preparation of the coating composition and application of the coating were carried out generally as described in Example 1.

The purpose of the following Examples 7-20, inclusive, is to illustrate the efiect of ultimate particle size and concentration (by weight) of particles upon the dielectric life of the coated film. In general, the examples summarized in Table II show that the amount of finely divided particles (by weight) in the resin/rubber composition must be reduced as the average particle size is reduced in order to obtain optimum dielectric life.

The following example illustrates the preparation of a high-density polyethylene film having an adherent coating of a silicone resin/silicone rubber/estersil composition.

A high-density (having a density of 0.955) polyethylene composition was melt extruded to form a film of 1 mil in thickness. This film was coated (in a manner similar to that described in Example 1) with the silicone resin/silicone rubbcr/estersil composition of Example 1 except that the curing catalyst employed was composed of 2/2% lead octoate and A2 of triethanolamine, based upon the total weight of the silicone resin. The resulting coating was dried and cured at room temperature, and the total coated film thickness was 2 mils.

The dielectric life of the coated film was greater than 50 hours at 1,000'volts/mil at room temperature (time to the 5th failure out of 10). The dielectric life of the uncoated film under the same conditions was 13 hours.

This application is a continuation-in-part of my copending application Serial No. 439,157 filed June 24, 1954, now abandoned.

I claim:

7 1. A polymeric, thermoplastic, dielectric base film having an adherent continuous coating of a cured homogeneous blend comprising 40-75% of a polymeric, organic, siloxane resin having a ratio of organic radicals-tosilicon of 1.0-1.7:1, 1540% of a polymeric, organic, siloxane rubber having a ratio of organic radicals-tosilicon of 1.982.0:1 and 520% of finely divided particles from the group consisting of amorphous silica and esterified super-colloidal substrates, said substrates having a specific surface area of at least 1 mcter /gram and a surface of silica, said silica coated with OR groups chemically bound thereto, R being a hydrocarbon radical of at least 2 carbon atoms wherein the carbon atom attached to oxygen is also attached to hydrogen.

2. A film as in claim 1 wherein the finely divided particles have an ultimate average particle size between 5 mil- 6. A film as in claim 1 wherein the polymeric, organic, siloxane rubber is a dimethyl siloxane rubber.

7. A fiim as in claim 1 wherein the polymeric, organic, siloxane resin is a methyl phenyl siloxane resin and the polymeric, organic, siloxane rubber is a dimethyl siloxane rubber.

8. A film as in claim 1 wherein the polymeric, thermoplastic, dielectric base film is polyethylene terephthalate.

9. A film as in claim 1 wherein the polymeric, thermoplastic, dielectric base film is balanced, heat-set polyethylene terephthalate film.

10. A coating composition tlor coating polymeric, thermoplastic, dielectric base film comprising a solution of a blend of 4075% of a polymeric, organic, siloxane resin having a ratio of organic radicals-to-silicon of 1.0-1.7:1, 15-40% of a polymeric, organic, siloxane rubber having a ratio of organic radicals-to-silicon of 1.982.0:1 and -20% of finely divided particles from the group consisting of amorphous silica and esterified super-colloidal substrates, said substrates having a specific surface area of at least 1 meter /gram and a surface of silica, said silica coated with OR groups chemically bound thereto, R being a hydrocarbon radical of at least 2 carbon atoms wherein the carbon atom attached to oxygen is also attached to hydrogen.

11. A process which comprises blending 40-75% of a polymeric, organic, siloxane resin having a ratio of organic radicals-to-silicon of 1.0-1.7:1, -40% of a polymeric, organic, siloxane rubber having a ratio of organic radicals-to-siliccn of 1.98-2.0:1 and 5-20% of finely divided particles from the group consisting of amorphous silica and esterified super-colloidal substrates, said sub strates having a specific surface area of at least 1 meter /gram and a surface of silica, said silica coated with OR groups chemically bound thereto, R being a hydrocarbon radical of at least 2 carbon atoms wherein the carbon atom attached to oxygen is also attached to hydrogen, in a volatile organic solvent for said siloxane resin and said siloxane rubber to form a coating composition;

14 coating a polymeric, thermoplastic, dielectric base film with said coating composition; drying the coated. film to remove the volatile organic solvent; and curing the resultant coating.

12. A process which complises blending a polymeric,

organic, siloxane resin having a ratio of organic radicalsto-si-licon of 1.0-1.7:1 with a polymeric, organic, siloxane rubber having a ratio of organic radicals-to-silicon of 1.98-2.0z1, said siloxane rubber containing finely divided particles from the group consisting of amorphous silica and esterified sup-ercolloidal substrates, said substrates having a specific surface area of at least 1 meter /gram' and a surface of silica, said silica coated with OR groups chemically bound thereto, R being a hydrocarbon radical of at least 2 carbon atoms wherein the carbon atom at-- tached to oxygen ,is also attached to hydrogen, in a volatile organic solvent for said siloxane resin'and said siloxane rubber in the presence of a chlorine-free phthalocyanine pigment to form a coating composition containing 4075% of the siloxane resin, 15-40% of the siloxane rubber and 5-20% of the finely divided particles; coating a polymeric, thermoplastic, dielectric base film with said coating composition; drying the coated film to remove the volatile organic solvent; and curing the resultant coating.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A POLYMERIC, THERMOPLASTIC, DIELECTRIC BASE FILM HAVING AN ADHERENT CONTINUOUS COATING OF A CURED HOMOGENEOUS BLEND COMPRISING 40-75% OF A POLYMERIC, ORGANIC, SILOXANE RESIN HAVING A RATIO OF ORGANIC RADICALS-TOSILICON OF 1.0-1.7:1, 15-40% OF A POLYMERIC, ORGANIC, SILOXANE RUBBER HAVING A RATIO OF ORGANIC RADICALS-TOSILICON OF 1.98-2.0:1 AND 5-20% OF FINELY DIVIDED PARTICLES FROM THE GROUP CONSISTING OF AMORPHOUS SILICA AND ESTERIFIED SUPER-COLLOIDAL SUBSTRATES, SAID SUBSTRATES HAVING A SPECIFIC SURFACE AREA OF AT LEAST 1 METER2/GRAM AND A SURFACE OF SILICA, SAID SILICA COATED WITH OR GROUPS CHEMICALLY BOUND THERETO, R BEING A HYDROCARBON RADICAL OF AT LEAST 2 CARBON ATOMS WHEREIN THE CARBON ATOM ATTACHED TO OXYGEN IS ALSO ATTACHED TO HYDROGEN. 